Metal processing method



Nov. 16, 1965 H. G. SELL ETAL 3,218,154

METAL PROCESSING METHOD Filed Dec. 6, 1962 FIG. 2.

t COMPACTED METAL POWDER FIG. I.

FIG. 3. 3s

38 FIG. 4.

FIG. 5.

COMPACT FINELY DIVIDED REFRACTORY METAL INTO A SELF-SUSTAINING ELONGATED MASS OF PRE- DETERMINED CROSS-SECTIONAL AREA HAVING A SUBSTANTIAL DEGREE OF POROSITY.

32 l SUPPORT SAID ELONGATED MASS IN A VACUUM. POWER L PACK BOMBARD A LIMITED LENGTH PORTION OF SAID ELONGATED MASS WITH AN ELECTRON BEAM To l 26 MELT SUCH BOMBARDED LENGTH PORTION.

; MOVE SAID ELONGATED MASS BI SAID ELECTRON {I 1 BEAM RELATIVE TO ONE ANOTHER AT A PRE- DETERMINED RATE OF SPEED TO SEQUENTIALLY BOMBARD BI MELT ADJACENT LIMITED LENGTHS OF SAID ELONGATED MASS.

CONTINUE THE MOVEMENT OF SAID ELECTRON BEAM ALONG THE LENGTH DIMENSION OF SAID ELONGAT ED '6 MASS UNTIL A PREDETERMINED PORTION OF SAID ELONGATED MASS HAS BEEN MELTED 8 THEN VACUUM SOLIDIFIED TO A BULK DENSITY SUBSTANTIALLY EQUAL TO THE THEORETICAL DENSITY OF THE REFRACTORY METAL.

INVENTOR5. HEINZ G. SELL and THOMAS MT: FALL.

Patented Nov. 16, 1965 e Eg 3,218,154 METAL PROCESSING METHQD Heinz G. Seil, Cedar Grove, and Thomas McFall, Binomfield, Ni, assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Dec. 6, 1962, Ser. No. 242,781 11 Claims. (Cl. 75--1t)) This invention relates to processing of refractory metals and, more particularly, to an improved method for consolidating finely divided, refractory metal into a unitary mass.

Electron-beam, zone-refining of refractory metals, such as tungsten or molybdenum is generally described in US. Patent No. 2,809,905, granted Oct. 15, 1957 to Davis et al. In accordance with the method described in this patent, a substantially solid bar of the metal to be refined is first prepared. This bar is then placed into an evacuated enclosure and the zone-refining process is carried out by sequentially melting zones of the bar with an electron beam. The resulting metal is limited in diameter (0.125 inch or less) but is quite pure and, in some cases, single crystals of the refined metal can be produced.

One of the drawbacks to the foregoing method is that the refractory metal, such as tungsten or molybdenum for example, must first be refined from the ore and reduced to finely divided metal powder. The metal powder is compacted into an ingot, the ingot presintered, and then self-resistance sintered or otherwise heated to a very high temperature, in order to cause the finely divided particles to coalesce into a unitary mass. Thereafter the resulting ingot is swaged and centerless ground to a bar of the desired dimensions, then surface cleaned for electron-beam, zone-refining purification.

Particularly during the swaging process, impurities are apt to be introduced into the ingot. Also, heating is fairly rapid in the usual self-resistance sintering technique, and this permits some impurities to be entrapped. In addition, some alloys of refractory metals are extremely brittle and readily fracture when mechanically reduced in size, such as by swaging. An example of such an alloy is tungsten containing about eight percent by weight of thoria. Heretofore, fabrication of parts comprising such alloys has been generally regarded as commercially impractical. Further, self-resistance sintering is normally accomplished in a hydrogen atmosphere and this atmosphere precludes formation of some alloys. An example of such an alloy is tungstentantalum, since tantalum forms a volatile hydride which is vaporized from the compact during the self-resistance sintering.

It is the general object of the present invention to provide a very simple method for coalescing finely divided particles of refractory metal into a unitary mass having a very high degree of purity and a bulk density which substantially corresponds to the theoretical density of the metal.

It is another object to provide a method for coalescing a mixture of finely divided refractory metal into an alloy of such metal, wherein the formed alloy has a very high degree of purity and a bulk density substantially corresponding to the theoretical density of the alloy.

It is a further object to provide a method of producing refractory metal which has a higher degree of purity than heretofore obtained.

It is an additional object to provide a simple method for making alloys of refractory metals which heretofore have been difiicult or impractical to make.

The aforesaid objects of the invention, and other objects which will become apparent as the description proceeds, are achieved by forming the finely divided refractory metal into a self-sustaining, elongated compact or mass. This compact is supported in a vacuum and a limited length portion of the supported compact is bombarded with an electron beam which has sufficient energy to melt such bombarded portion. Since only a limited length portion of the compact is bombarded, the surface tension of the melted material prevents it from running. The bombarding electron beam is moved along the length of the elongated compact at such rate of speed as to sequentially melt adjacent limited length portions of the compact with melted portions of the compact solidifying after bombardment. The resulting solidified material has a bulk density which is substantially equal to the theoretical density of the metal. This melting and solidifying procedure is continued until a predetermined, desired length of the compact has been melted and then solidified. The resulting material will frequently have a single crystal structure and single crystal alloys of refractory metal can also be produced by this method. There is also provided the resulting metal which has been processed by the foregoing method.

For a better understanding of the invention, reference should be had to the accompanying drawing, wherein:

FIG. 1 is a schematic view of an electron-beam, zonerefining furnace;

FIG. 2 is a perspective view of a porous, self-sustaining compact of compacted refractory metal powder;

FIG. 3 is a perspective view of the compact as shown in FIG. 2, but with end supports added, preparatory to refining;

FIG. 4 is a perspective view of the finished metal bar after refining in accordance with the present method; and

FIG. 5 is a flow chart which sets forth the essential steps of the present method.

With specific reference to the form of the invention illustrated in the drawings, the electron-beam, zone-refining furnace 10 as shown in FIG. 1 generally comprises a shell 12 which is hermetically closed at both ends by flanges 14 and 16. A threaded spindle 18 extends into the furnace 10 through a vacuum-tight bushing 20. A frame 22 is slidably mounted within the furnace shell 12 on a pair of vertical guide rods 24-, and the upper end of the sliding frame 22 is affixed to the threaded spindle 18. A pair of brackets 26 are affixed to the upper and lower ends of the sliding frame 24, in order to support the metal compact 28 which is being treated. A worm wheel and worm drive 30 connects to the threaded spindle 18 and is driven at a uniform speed by a motor drive (not shown) to reciprocate the threaded spindle 18 at a predetermined, uniform rate, with the frame 22 moving up and down on the guide rods 24. The volume enclosed by the furnace is adapted to be evacuated by a conventional vacuum system.

The electron gun comprises a circular cathode 32, which is formed of tungsten, and is adapted to encircle the compact 28 which is to be sintered. The circular cathode 32 is electrically connected to a power pack 34 and the compact 28 comprises the anode or target for the electron beam. Thus the compact 28 is moved relative to the fixed gun 32. The uppermost and lowermost positions of the movable compact 28 are shown in dotted lines in FIG. 1. In the stage of refining as shown, the compact has already moved the distance a and will move the distance :1 before completing one pass through the electron gun 32. While the power pack 34 has been shown only in block form, a power pack suitable for operation with the present invention is disclosed in copending application Ser. No. 118,389, filed June 20, 1961, and owned by the present assignee.

As a specific example, the purification and processing of tungsten metal powder will be considered in detail.

Finely divided tungsten metal powder is first refined from the ore in accordance with conventional practices to as pure a state as possible. This finely divided metal powder is then placed into a mold and formed into a compact 28 of such finely divided material, as shown in FIG. 2. The dimensions of the compart are A inch square and twelve inches long, and the compacting pressure which is used in forming the compact is about 20,000 p.s.i. The bulk density of the formed compact is about 60% of the theoretical density of tungsten, which is 19.3. The formed compact is self-sustaining in nature in that it can be handled without breakage and the pressed compact could be zone refined without any preliminary sintering. It is preferred, however, to heat the formed compact at a relatively low temperature to provide some degree of incipient sintering between the particles comprising the compact to facilitate handling. As a specific example, the formed compact is heated to a temperature of 1000 C. for a period of 30 minutes in a hydrogen atmosphere, or in vacuum in the case of alloys.

To prepare the compact 28 for processing in accordance with the present invention, small holes 36 are drilled into the ends of the compact 28 and metal support rods 38 inserted therein, as is shown in FIG. 3. The compact 28 is then placed into the electron beam furnace 10, as shown in FIG. 1, and the compact is supported proximate to its ends by inserting the metal bars 38 into the retaining brackets 26. The furnace 10 is then evacuated to a pressure of l mm. Hg or less, for example.

Preferably the compact is mounted vertically in the furnace since there is less tendency for the metal comprising the melted zone to run. Prior to actual zone refining, the end portions of the compact 28 desirably are heated by the electron beam to a temperature slightly below the melting temperature of the metal, in order to effect a sintering between the metal support rods 38 and the compact 28. The compact 28 is then moved in a first pass through the electron gun 32 at a rate of approximately five millimeters per minute, but with a power input level lower than that required for melting. As a specific example, the power input used during this first pass is approximately 90% of that power as is required for melting the metal. This first pass facilitates outgassing the pourous compact and also effects some sintering. The power level is then raised to a predetermined input, as is required for melting, and a molten zone is formed at one end of the compact. This molten zone is caused to travel the length of the compact 28 by moving the compact through the electron gun 28, with the rate of movement being about five millimeters per minute. The power input which is required to melt a limited section of the foregoing specific compact requires a potential of approximately 2500 volts and a current of 0.5 ampere. Upon melting, the square cross section of the compact changes to a circular cross section having an average diameter of about 0.22 inch, and a variation in diameter of about 0.01 inch can be expected. The section of the compact which is molten at any one time has a length of about 0.25 inch.

The finished processed bar or rod 40 is shown in FIG. 4. After processing, the ends or end which contain the impurities are removed. The resulting metal has an exextremely high degree of purity and a bulk density which is about 99.99% that of the theoretical density of tungsten. In addition, in the case of refractory metals such as tungsten and molybdenum, the resulting formed material is one continuous crystal with the same crystallographic orientation throughout. The essential steps of the foregoing method are outlined in the flow chart as shown in FIG. 5.

While tungsten has been considered in detail in the foregoing example, other metals which have melting points greater than 1100 C. can also be processed in accordance with this method. Examples of such other metals are molybdenum, niobium, tantalum, rhenium, osmium, ru-

thenium, platinum, yttrium, vanadium, chromium and iron. It should be noted that the higher the melting temperature of the metal being processed the greater the resulting purity. In explanation, when refining tungsten, which has a melting point of approximately 3370 C., unwanted impurities will have a high vapor pressure at this very high melting temperature, and will be more readily volatilized during the refining. Those impurities which are soluble in the melted material will of course be carried by the zone refining process to one end section of the compact 28 where they are later removed by cutting olf the impure end section.

The extreme purity of the metal which has been processed in accordance with the present method is best illustrated by the residual resistance ratio of the resulting metal. In explanation of this term, the residual resistance ratio is the ratio of the electrical resistance at room temperature divided by the electrical resistance at liquid helium temperature. The higher this ratio, the purer the metal. In the case of tungsten which has been processed in accordance with the present method, the residual resistance ratios obtained are in excess of 35,000. The residual resistance ratio for tungsten purified by electronbeam, zone-refining a sintered and swaged tungsten rod can vary from 5,600 to a maximum of 14,000. The residual resistance ratio obtained with tungsten prepared in accordance with conventional powder metallurgy techniques is about 200. The residual resistance ratio for molybdenum processed in accordance with the present method is about 4200. The residual resistance ratio for molybdenum which is processed in accordance with usual powder metallurgy techniques is from about 50 to 80.

The present method also permits a better control of the melted section of the elongated mass being processed. In explanation, a porous compact, which has a density of approximately 60% that of theoretical, is a relatively poor conductor of heat. Thus there is less heat conducted from the zone which is being melted and this makes the melted zone more stable and more readily controlled. This is in addition to the other advantages of refining a porous compact rather than a bar of solid metal.

In the preferred method, at least two passes through the electron gun are desired, as specified hereinbefore. The first pass, which is made with a power input less than that required to melt the compact, is very effective in causing the volatile impurities to evaporate. If only one pass was used, the material would be densified more rapid- 1y, thereby eliminating substantially all voids in a fairly rapid fashion. This would limit the volatilization of some of the impurities. It should be understood, however, that only one pass can be used if desired, in which case the very porous compact would be melted and converted to solidified material during only one pass.

The present method is extremely useful for making alloys of refractory metals, and with this method it is possible to produce alloys which heretofore have not been practical to produce because of swaging or other difliculties. As an example, alloys of tungsten-niobium, tungsten-tantalum, tungsten-molybdenum, tungsten-rhenium, molybdenum-rhenium and molybdenum-niobium can readily be produced by the present process. When producing alloys by the present process, it is preferred to use at least two passes of the compact through the electron gun, with the directions of each pass being opposite. In explanation, during the melting, there may be some tendency to increase the concentration of one of the alloying ingredients at the melted zone. This will have the efiect of carrying one of the ingredients comprising the alloy to one end of the compact. When remelting by passing the elongated mass through the gun in the opposite direction, the concentration of the alloys within the mass is made quite uniform, so that the alloy in effect is zone leveled. When processing a single metal in accordance with the present method, however, and when using more than one pass, it is preferred that all passes through the electron gun be made in the same direction, in order that all impurities will be concentrated in one end of the elongated mass, thereby minimizing the amount of processed material which later is discarded.

In a preferred form of the present method, the elongated mass or compact of refractory metal is supported at longitudinally spaced locations in a vertical position. Such an orientation is preferred since the melted zone or length portion of the compact has less tendency to run. As a possible alternative embodiment, the compact could be suspended in a horizontal position with both ends firmly fixed and the melted zone limited in size so that its surface tension would prevent the melted metal from running.

Alloying materials which are present in the concentration range of solid solubility, and most single metals, can be readily formed into single crystals when they are processed in accordance with the present method. In the case of metal such as iron which undergoes phase transformations, however, some difiiculties in producing single crystals are encountered. Dispersed second phase alloys can be produced when the second phase is stable at the melting temperature of the alloy.

It is preferred to use a pressure-compacting technique when preparing the elongated mass. The compact can be otherwise fabricated, however, such as by using'a slipcasting technique. A suitable slip-casting medium is isobutyl acetate or o-xylene.

Summarizing the present method, the finely divided metal to be refined is first formed in a self-sustaining, elongated mass, which has a substantial degree of porosity. This mass is supported in a vacuum at longitudinally spaced locations and a limited length portion of the mass is bombarded with an electron beam which has such predetermined energy as required to melt a limited length portion of the mass. The surface tension of the melted metal keeps the melt from running from the mass. The elongated mass and bombarding electron beam are moved relative to one another between the supported locations on the mass and at a predetermined rate of speed to sequentially melt adjacent limited length portions of the mass, with melted portions solidifying after bombardment. This sequential bombardment, melting and cooling of limited length portions of the mass is continued until a predetermined length of the mass has been melted and then solidified.

The specific example as given hereinbefore considers in detail a tungsten compact having specific dimensions. It should be understood that varying the size of the compact will require varying the processing conditions. Also, varying the metal being processed will require varying the processing conditions. The proper processing conditions for any specific porous mass can be readily determined. For example, when processing a porous molybdenum compact having dimensions the same as those of the tungsten compact previously considered, the voltage is 2500 and the current is 0.3 ampere. For processing a porous compact formed of tungsten and 5% tantalum, the compact is first vacuum presintered at a temperature of 1050" C. and other processing conditions are the same as specified for the tungsten compact previously considered.

It will be recognized that the objects of the invention have been achieved by providing a simple method for processing single refractory metals or refractory metal alloys. The resulting product has extremely high purity and has a bulk density substantially equal to that of the theoretical density of the metal. This process enables alloys to be fabricated which have heretofore been extremely difficult or impractical to fabricate using conventional equipment.

While a specific example of the invention has been illustrated and described hereinbefore, it is to be particularly understood that the invention is not limited thereto or thereby.

We claim as our invention:

1. The method of consolidating finely divided refractory metal into a unitary mass having a high bulk density substantially corresponding to the theoretical density of such refractory metal, which method comprises:

(a) forming said finely divided metal into a substantially unsintered self-sustaining elongated mass of predetermined cross-sectional area and having a substantial degree of porosity;

(b) supporting said elongated mass in a vacuum;

(c) bombarding a limited length portion of said mass with an electron beam having predetermined energy as required to melt such bombarded portion throughout its cross section, with the limited length of such bombarded portion preventing the melted metal from running;

(d) moving said elongated mass and said bombarding electron beam relative to one another to cause said electron beam to move at a predetermined rate of speed along the length dimension of said mass to sequentially melt adjacent limited length portions of said mass, with melted portions of said mass solidifying after bombardment to a bulk density substantially equal to the theoretical density of said metal; and

(e) continuing the movement of said electron beam along the length dimension of said mass until a predetermined length of said mass has been melted and then solidified to a bulk density substantially equal to the theoretical density of said metal.

2. The method of consolidating finely divided refractory metal into a unitary mass having a high bulk density substantially corresponding to the theoretical density of such refractory metal, which method comprises:

(a) forming said finely divided metal into a self-sustaining elongated mass of predetermined cross-sectional area and having a substantial degree of porosity and which mass at most is only sintered sufficiently to permit handling;

(b) supporting said elongated mass at longitudinally spaced locations and in a vacuum;

(0) bombarding a limited length portion of said mass which is positioned between the spaced locations at which said mass is supported with an electron beam having predetermined energy as required to melt such bombarded portion throughout its cross section, with the limited length of such bombarded portion preventing the melted metal from running;

((1) moving said elongated mass and said bombarding electron beam relative to one another to cause said electron beam to move at a predetermined rate of speed along the length dimension of said mass between the supported locations thereof to sequentially melt adjacent limited length portions of said mass, with melted portions of said mass solidifying after bombardment to a bulk density substantially equal to the theoretical density of said metal; and

(e) continuing the movement of said electron beam along the length dimension of said mass between supported locations thereof until a predetermined length of said mass has been melted and then solidified to a bulk density substantially equal to the theoretical density of said metal.

3. The method as specified in claim 2, wherein said elongated mass is moved in said vacuum at a predetermined rate with respect to said electron beam.

4. The method as specified in claim 2, wherein prior to melting said elongated mass, said mass and said electron beam are moved relative to one another to cause said mass to be bombarded with sutficient energy to effect sintering but not melting of said mass.

5. The method as specified in claim 2, wherein said refractory metal is one metal of the group consisting of tungsten and molybdenum.

6. The method as specified in claim 2, wherein the longitudinally spaced locations at which said elongated mass is supported in a vacuum are proximate to the end portions of said elongated mass.

7. The method as specified in claim 6, wherein metallic members are afiixed to the end portions of said elongated mass, and said elongated mass is supported by securing said metallic members.

8. The method of consolidating dilferent mixed finely divided refractory metals into a unitary alloyed mass having a high bulk density substantially corresponding to the theoretical density of an alloy formed of such different refractory metals, which method comprises:

(a) forming said finely divided metals into a selfsustaining elongated mass of predetermined crosssectional area and having a substantial degree of porosity and which mass at most is only sintered sufficiently to permit handling;

(b) supporting said elongated mass at longitudinally spaced locations and in a vacuum;

(c) bombarding a limited length portion of said mass which is positioned between the spaced locations at which said mass is supported with an electron beam having predetermined energy as required to melt such bombarded portion throughout its cross section, with the limited length of such bombarded portion preventing the melted metal from running;

(d) moving said elongated mass and said bombarding electron beam relative to one another to cause said electron beam to move in a first direction at a predetermined rate of speed along the length dimension of said mass between the supported locations there of to sequentially melt adjacent limited length portions of said mass, with melted portions of said mass solidifying after bombardment;

(e) continuing the movement of said electron beam along the length dimension of said mass between supported locations thereof until a predetermined length of said mass has been melted and then solidified; and

(f) repeating the foregoing melting and soidifying operations on said elongated mass by moving said mass and said electron beam relative to one another in a direction which is opposite to the first direction in which said mass and said beam were previously moved relative to each other to insure an even distribution of the different refractory metals comprising the formed alloy.

9. The method as specified in claim 8, wherein prior to melting said elongated mass, said mass and said electron beam are moved relative to one another to cause said mass to be bombarded with sufiicient energy to effect sintering but not melting of said mass.

10. The method of consolidating a single finely divided refractory metal into a unitary mass having a high bulk density substantially correspnding to the theoretical density of such refractory metal, which method comprises:

(a) pressure compacting said finely divided metal into a self-sustaining elongated mass of predetermined cross-sectional area and having a substantial degree of porosity and which compact at most is only sintered sufficiently to permit handling;

(b) supporting said elongated mass at longitudinally spaced locations and in a vacuum;

() bombarding said elongated mass throughout its length dimension with an electron beam having such energy as to effect sintering but not melting of said mass;

((1) bombarding a limited length portion of said mass which is positioned between the spaced locations at which said mass is supported with an electron beam having predetermined energy as required to melt such bombarded portion throughout its cross sction, with the limited length of such bombarded portion preventing the melted metal from running;

(e) moving said elongated mass and said bombarding electron beam relative to one another to cause said electron beam to move at a predetermined rate of speed along the length dimension of said mass between the supported locations thereof to sequentially melt adjacent limited length portions of said mass, with melted portions of said mass solidifying after bombardment to a bulk density substantially equal to the theoretical density of said metal;

(f) continuing the movement of said electron beam along the length dimension of said mass between supported locations thereof until a predetermined length of said mass has been melted and then solidified to a bulk density substantially equal to the theoretical density of said metal; and

(g) repeating the foregoing melting and solidifying operations on said elongated mass by moving said mass and said electron beam relative to one another in a direction which is the same as that direction in which said mass and said beam were previously moved relative to each other.

11. A unitary mass of refractory metal having a very high degree of purity and a bulk density substantially corresponding to the theoretical density of such metal, said metal having been produced by the method which comprises:

(a) forming said metal when in finely divided form into a self'sustaining elongated mass of predetermined cross-sectional area and having a substantial degree of porosity and which mass at most is only sintered sufiiciently to permit handling;

(b) supporting said elongated mass in a vacuum;

(c) bombarding a limited length portion of said mass with an electron beam having predetermined energy as required to melt such bombarded portion throughout its cross section, with the limited length of such bombarded portion preventing the melted metal from running;

(d) moving said elongated mass and said bombarding electron beam relative to one another to cause said electron beam to move at a predetermined rate of speed along the length dimension of said mass to sequentially melt adjacent limited length portions of said mass, with melted portions of said mass solidifying after bombardment to a bulk density substantially equal to the theoretical density of said metal; and

(e) continuing the movement of said electron beam along the length dimension of said mass until a predetermined length of said mass has been melted and then solidified to a bulk density substantially equal to the theoretical density of said metal.

References Cited by the Examiner UNITED STATES PATENTS 2,743,199 4/1956 Hull -65 2,809,905 10/1957 Davis 7563 2,904,411 9/1959 Pfann 75-l0 3,163,523 12/1964 Porter 75-84 OTHER REFERENCES Moore et al.: Preparation of High Purity W, Mo, Ta, Cb and Zr, Wright Air Development Center Report 59 314, October 1959, pages 19,

DAVID L. RECK, Primary Examiner.

WINSTON A. DOUGLAS, BENJ'AMEN HENKIN,

Examiners 

1. THE METHOD OF CONSOLIDATING FINELY DIVIDED REFRACTORY METAL INTO A UNITARY MASS HAVING A HIGH BULK DENSITY SUBSTANTIALLY CORRESPONDING TO THE THEORETICAL DENSITY OF SUCH REFRACTORY METAL, WHICH METHOD COMPRISES: (A) FORMING SAID FINELY DIVIDED METAL INTO A SUBSTANTIALLY UNSINTERED SELF-SUSTAINING ELONGATED MASS OF PREDETERMINED CROSS-SECTIONAL AREA AND HAVING A SUBSTANTIAL DEGREE OF POROSITY; (B) SUPPORTING SAID ELONGATED MASS IN A VACUUM; (C) BOMBARDING A LIMITED LENGTH PORTION OF SAID MASS WITH AN ELECTRON BEAM HAVING PREDETERMINED ENERGY AS REQUIRED TO MELT SUCH BOMBARDED PORTION THROUGHOUT ITS CROSS SECTION, WITH THE LIMITED LENGTH OF SUCH BOMBARDED PORTION PREVENTING THE MELTED METAL FROM RUNNING; (D) MOVING SAID ELONGATED MASS AND SAID BOMBARDING ELECTRON BEAM RELATIVE TO ONE ANOTHER TO CAUSE SAID ELECTRON BEAM TO MOVE AT A PREDETERMINED RATE OF SPEED ALONG THE LENGTH DIMENSION OF SAID MASS TO SEQUENTIALLY MELT ADJACENT LIMITED LENGTH PORTIONS OF SAID MASS, WITH MELTED PORTIONS OF SAID MASS SOLIDIFYING AFTER BOMBARDMENT TO A BULK DENSITY SUBSTANTIALLY EQUAL TO THE THEORETICAL DENSITY OF SAID METAL; AND (E) CONTINUING THE MOVEMENT OF SAID ELECTRON BEAM ALONG THE LENGTH DIMENSION OF SAID MASS UNTIL A PREDETEREMINED LENGTH OF SAID MASS HAS BEEN MELTED AND THEN SOLIDIFIED TO A BULK DENSITY SUBSTANTIALLY EQUAL TO THE THEORETICAL DENSITY OF SAID METAL. 